Intravascular blood parameter sensing system

ABSTRACT

A parameter of blood is sensed in vivo with a system which includes a catheter and a probe. The catheter has a lumen extending therethrough, a proximal end, a distal end and a distal opening at the distal end. The probe includes one or more sensing elements at its distal end. In one embodiment, the sensing element or elements are located at least about 4 mm proximal of the distal opening of the lumen. A volume oscillator is provided to control the movement of blood into and out of the lumen. A flush solution is introduced into the lumen so that when the volume oscillator is inactive the sensing element or elements are exposed to pure flush solution, which preferably includes an anti-clotting component. The probe is received within the catheter and affixed thereto. The volume oscillator is activated to move blood into and out of the lumen to expose the sensing element or elements to blood so that they can sense the blood parameter or parameters of interest.

BACKGROUND OF THE INVENTION

The present invention relates to systems for sensing or measuring one ormore parameters of blood from a blood vessel of a patient. Moreparticularly, this invention relates to assemblies and methods usefulfor sensing a parameter of blood in vivo involving a catheter and asensor probe a portion of which is located in a lumen of the catheter.

Sensors and sensor probes are well known, for example, for use indetermining the concentrations (meaning to include therein the partialpressures of gases) of blood analytes or constituents of interest.Typically of interest is the determination of the concentration ofgases, such as oxygen and carbon dioxide, of hydrogen ions (pH), ofother electrolytes, of glucose and the like in the blood. These provideuseful parameters for assessment of certain physiological conditions ofa mammal, for example, a human medical patient.

Such sensors and sensor probes can be made sufficiently small in size tobe used directly in vivo in a patient. This contributes to monitoringthe condition of the patient on a continuous basis as opposed to olderknown procedures which require the extraction of a blood sample fordetermination in a remote laboratory of the constituents of interest inthe blood.

An example of a useful blood sensor involves an optical indicator, suchas a fluorescence indicator, located in association with a matrixmaterial, such as a polymeric matrix material, on the optical face orsurface of an optical fiber. Typically, a matrix material containing afluorescent dye is loaded onto the optical surface of an optical fiber.Interaction of the dye with the component of interest, that is thecomponent to be sensed, is monitored using optical signals carried bythe optical fibers.

Maxwell U.S. Pat. No. 4,830,013 discloses in vivo sensing of variousblood parameters using a sensor probe which is adapted to be insertedthrough a catheter into the cardiovascular system of a patient. A sensorprobe is provided and preferably includes a plurality of sensors, eachof which includes a sensor element adapted for sensing a parameter ofblood and for providing a signal in response thereto and elongatedtransmission means, for example, an optical fiber, for transmitting thesignal from the sensor element proximally.

The above-noted Maxwell patent discloses that the sensor is placedbetween 0.005 inch and 0.125 inch proximal of the distal opening of thelumen. An anti-clotting solution is provided through the lumen of thecatheter to the patient. The combination of this solution and blood inthe lumen creates an interface zone including both blood andanti-clotting solution. This interface zone is moved over the sensor sothat the sensor can be bathed in blood for a period of time sufficientto enable the sensor to provide a signal relating to the blood parameterof interest, and to allow the anti-clotting solution to be expelled fromthe distal opening of the lumen. Thus, the sensor is either exposed tosubstantially exclusively blood or to a combination of blood andanti-clotting solution in the interface zone.

A related system is disclosed in Maxwell et al U.S. Pat. No. 4,951,669in which the sensors are located in the fitting proximal of thecatheter. In one embodiment, the sensors are shown in the lumen near theenlarged proximal end of the catheter. This system, which can beoperated very similarly to the system in the above-noted Maxwell patent,is designed to allow the use of bigger sensor elements. Both theabove-noted Maxwell and Maxwell et al patents are incorporated in theirentireties herein by reference.

Carbon dioxide and relatively high concentrations of oxygen are bloodparameters which are quite difficult to measure accurately.

It clearly would be advantageous to provide blood parameter sensingsystems which provide enhanced accuracy.

SUMMARY OF THE INVENTION

New assemblies and methods for sensing parameters of blood have beendiscovered. The present invention is based, in part, on the discoverythat the location of the sensor (sensing element) in the catheter isimportant to control the environment to which the sensor is exposed. Bycontrolling this environment, it has been found that the accuracy of theblood parameter measurements obtained is benefitted. Moreover,substantial additional advantages, for example, in terms of ease ofcalibration and reduced thrombus formation, are achieved. Further, a newvolume oscillator is provided which very effectively and efficientlycontrols the movement of blood into and out of the catheter. Blood ismoved, preferably in a predetermined time sequence, by the volumeoscillator to at least facilitate obtaining certain advantages achievedby the present system. Also, the volume oscillator is preferablystructured to obtain added advantages, such as safety, disposability,predictability, cost effectiveness and reduced interference with otherparts of the system. In short, the present systems effectively andefficiently provide accurate measurements of blood parameters.

In one broad aspect, the present invention is directed to assemblies forsensing blood parameters which comprise a probe and a catheter. Theprobe includes a sensing element for sensing a parameter of blood andfor providing a signal in response to the sensed parameter, and anelongated transmission member for transmitting the signal from thesensing element. The catheter includes a lumen in which the sensingelement is located. The lumen has an opening in the distal end. Thecatheter is sized and adapted so that at least the distal end and theopening in the distal end are receivable within a blood vessel of apatient.

In one embodiment, the sensing element is located in the lumen, forexample, away from the proximal end of the catheter and/or closer to thedistal opening than to the proximal end of the catheter, and at leastabout 4 mm, preferably at least about 6 mm, proximal of the distalopening. The sensing element is preferably located up to about 40 mm,more preferably up to about 20 mm, proximal of the distal opening of thelumen. By fixing the location of the sensing element as noted above, ithas been found that the environment to which the sensing element isexposed can be controlled substantially independently of the tidalaction caused by the heartbeat of the patient, and that only arelatively small amount of blood need be moved from the blood vessel ofthe patient into the catheter to expose the sensing element tosubstantially exclusively blood, for example, pure blood. Substantiallyindependent control of the environment to which the sensing element isexposed facilitates the increased accuracy of the present systemrelative to, for example, the above-noted Maxwell system in which theheartbeat induced tidal action acted to move the blood back and forthover the sensors. As is disclosed hereinafter, the present systemadvantageously reduces the risks of blood stagnation and/or thrombusformation.

As used herein, the term "substantially exclusively" refers to a mediumwhich effectively has the same average value of the parameter ofinterest as the pure medium. Thus, "substantially exclusively blood" ispure blood which may include another component, e.g., flush or dripsolution, in an amount which is not effective to change the measuredaverage value of the parameter of interest relative to the measuredvalue of the parameter of interest in pure blood.

The movement of blood into and out of the lumen of the catheter ispreferably controlled substantially solely by a volume oscillator influid communication with the lumen of the catheter. Thus, when thevolume oscillator is inactive, the position of the sensing element inthe lumen is such that the heartbeat of the patient, the size of theblood vessel in which the catheter is located, the compliance of thepresent system (other than the compliance provided by the volumeoscillator itself), and all other environmental factors are insufficientto cause blood from the blood vessel of the patient to reach the sensingelement. With the volume oscillator inactive, the sensing element ispreferably exposed to substantially exclusively flush solution whichpasses through the lumen and into the blood vessel of the patient, at arelatively low rate, to keep the lumen patent. In other words, with thevolume oscillator inactive, the sensing element is preferably removed orseparated from both pure blood and the blood/flush solution interfacezone.

In one embodiment, the sensing element is located so as to be exposed toor in substantially exclusively a non-blood-containing fluid medium, forexample, the above-noted flush solution, during a portion of the timethe sensing element provides the parameter sensitive signal. It has beenfound that monitoring the parameter sensitive signal even while thesensing element is located in substantially exclusively anon-blood-containing fluid medium unexpectedly provides improvedaccuracy for at least certain of the blood parameters in question, forexample, the carbon dioxide concentration of the blood and forrelatively high oxygen concentrations, for example, oxygen partialpressures of about 90 mm Hg or higher, of the blood. Thus, the sensingelement provides a signal when the sensing element is exposed tosubstantially exclusively blood and when the sensing element is exposedsubstantially exclusively to a non-blood-containing fluid medium.Preferably, the time during which the sensing element is in or exposedto the blood/flush solution interface zone is reduced, more preferablyminimized. It has been found that the value of the parameter of interestin the non-blood-containing fluid medium is often known and, therefore,can be accounted for relatively easily. The value of the parameter ofinterest in the interface zone is often ambiguous and variable, and isnot easily accounted for. By reducing or minimizing the time duringwhich the sensing element is exposed to this ambiguous interface zone,the accuracy of the measurement of the parameter of interest in pureblood can be increased.

The pure blood parameter value is obtained by monitoring the value ofthe parameter of interest when the sensing element is exposed to bothsubstantially exclusively blood and substantially exclusivelynon-blood-containing fluid medium having a known value of the parameterso as to provide an average parameter value. The contribution to thismonitored average value provided by the substantially exclusivelynon-blood-containing fluid medium is then factored out to arrive at thevalue of the parameter in pure blood. This approach provides pure bloodparameter values which are more accurate than values obtained while thesensing element (during the time a parameter sensitive signal is beingprovided) is exposed only to substantially exclusively blood and theblood/flush solution interface zone.

Having the sensing element located as described herein provides one ormore additional substantial advantages. For example, the sensing elementcan be easily calibrated using a one point system, that is calibratingthe sensing element based upon the known volume of the parameter ofinterest in the non-blood-containing fluid medium, e.g., pure flushsolution. Since, with the volume oscillator inactive, the sensingelement is preferably exposed substantially exclusively to the flushsolution, which preferably includes an anti-clotting component such asheparin, the tendency to thrombus formation at or near the distal tip ofthe probe is reduced. In addition, because pure blood and theblood/flush solution interface zone are located outside the lumen orvery near the distal end of the lumen when the volume oscillator isinactive, the tendency for blood to stagnate in the lumen is alsoreduced. Moreover, when the volume oscillator is activated, it ispossible to "wash" or "flush" the sensing element with the flushsolution periodically. This again reduces the tendency to thrombusformation.

In another broad aspect of the present invention, volume oscillatorswhich are adapted to be placed in fluid communication with a lumen of acatheter, for example, as described above, are provided. These volumeoscillators are positioned so as to control the flow of blood from ablood vessel of a patient into and out of the lumen. In one embodiment,such volume oscillators comprise a housing defining a fluid flow path, apiston moveable relative to the housing and together with the housingdefining a chamber of variable volume, a stem secured to the piston andadapted to be activated to move the piston relative to the housing tovary the volume of the chamber, and an actuator adapted to be removeablycoupled to the stem and to activate the stem.

Preferably, the stem is activated in a predetermined time sequence sothat the volume of the chamber varies with time in accordance with awave form other than a simple sine wave. Previous systems have employeda simple sine wave form to control the movement of a piston. However,such a simple sine wave form provides that the sensing element isexposed to substantially exclusively blood for only a relatively smallfraction of the total time the volume oscillator is activated orrequires that a relatively large amount of blood be moved into and outof the lumen. This sine wave activation of the volume oscillator alsoexposes the sensing element to the ambiguous blood/flush solutioninterface zone for a relatively large fraction of the time the volumeoscillator is activated. The present volume oscillator is morepreferably activated in accordance with a wave form so that the fractionof the total time the volume oscillator is active during which thesensing element is exposed to substantially exclusively blood isincreased and/or the fraction of the total time the volume oscillator isactive during which sensing element is exposed to the blood/flushsolution interface zone is decreased, relative to the volume oscillatorbeing activated in accordance with a simple sine wave.

A particularly useful class of wave forms are those which provide thatthe amount of blood in the lumen of the catheter is maintainedsubstantially constant for a portion, more preferably a major portion,of the time the volume oscillator is activated. Such wave forms reduce,and even eliminate, any interference caused by the action of the volumeoscillator on the measurement of the patient's blood pressure throughthe lumen. A true and accurate blood pressure can be measured throughthe lumen with the volume oscillator activated when the amount of bloodin the lumen is maintained substantially constant.

In one embodiment, the housing, piston and stem are adapted to bedisposed of after use with a single patient. In this manner, improvedpatient safety is achieved. Also, since the actuator is adapted to beremoveably secured to the stem, it can be repeatedly used. The actuatoroften includes the more costly components of the present volumeoscillator assemblies. Therefore, by disposing of certain componentswhich may be exposed to blood after a single use while repeatedly usingother, more costly components, the present volume oscillators providefor patient safety and isolation in a cost effective manner.

The actuator preferably includes a motor with a motor shaft which isadapted to be removeably coupled to the stem. A control system isprovided which is adapted and positioned to provide control informationto the motor to control the motor and thereby control the time sequencein which the stem is activated. This control system preferably includesa microprocessor which is adapted to be preprogrammed to provide thecontrol information.

In one particularly useful embodiment, the present volume oscillatorassemblies further comprise a holder sized and adapted to carry thehousing and at least a portion of the actuator, for example, the motor.This holder, which preferably is used repeatedly (and not disposed ofafter use with a single patient), includes a cavity sized and adapted toreceive and hold the housing. The holder and housing include positioningelements which are adapted to be mutually engaged to hold the housing inthe cavity. In this manner, the disposable components of the presentsystem can be placed in and removeably secured to the holder for use.After use, the disposable components of the present assemblies areremoved, suitably disposed of, and replaced by a new disposablesub-assembly for use with the same patient or a different patient.

Methods for sensing a parameter of blood are provided. In oneembodiment, these methods comprise providing a catheter, such asdescribed herein, with the distal opening being positioned within theblood vessel of a patient. A probe, such as described herein, isprovided at least partially within the lumen of the catheter so that thesensing element is located as described herein. A flush, e.g., drip,solution from a flush solution source is introduced into the lumen. Avolume oscillator which, when activated, moves blood from the bloodvessel of the patient into and out of the lumen is provided. A signalresponsive to the parameter of interest is obtained from the sensingelement while the sensing element is exposed to substantiallyexclusively blood. The volume oscillator preferably acts to provide thatthe sensing element is exposed to substantially exclusively blood for amajor portion of the time the volume oscillator is activated. In aparticularly useful embodiment, the sensing element is exposed tosubstantially exclusively the flush solution for another portion of thetime the volume oscillator is activated. In this embodiment, theparameter sensitive signal is preferably also obtained while the sensingelement is exposed to substantially exclusively the flush solution.

The probe may carry one or more sensing elements depending upon thenumber of parameters of interest. These sensing elements can be of anytype, such as electro-chemical, that is suitable for sensing theparameter of interest; however, optical sensing elements are preferred,and fluorescent sensing elements are considered optimum. Althoughmultiple sensing elements could be provided to sense the same bloodparameter, preferably, each sensing element senses a different bloodparameter. In a preferred construction, the transmission member includesan optical fiber for each of the sensing elements, with the sensingelement being located on the distal end of the associated optical fiber.The sensing elements provide signals related to the associated bloodparameters of interest, and such signals may be used or processedcontinuously, intermittently or on demand, preferably continuously whilethe volume oscillator is activated, to provide readings indicative ofthe blood parameters of interest.

The invention, together with additional features and advantages thereof,may best be understood by reference to the following description takenin connection with the accompanying illustrative drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an assembly for the in vivo measurement ofblood parameters of interest.

FIG. 2 is an enlarged fragmentary sectional view of the distal region ofone form of probe and catheter usable in the assembly of FIG. 1.

FIG. 3 is a longitudinal sectional view through the probe-catheterassembly.

FIG. 4 is an enlarged sectional view taken generally along line 4--4 ofFIG. 2.

FIG. 5 is a generalized schematic illustration of the volume oscillatorassembly employed in FIG. 1.

FIG. 6 is a perspective view of certain disassembled components of thevolume oscillator.

FIG. 7 is a cross-sectional view of the volume oscillator showing thepiston stem in a substantially fully extended position.

FIG. 8 is a front plan view of the volume oscillator showing the pistonstem in a substantially fully retracted position.

FIG. 9. is a side view, partly in cross-section, of the volumeoscillator.

FIG. 10 is a schematic graphical illustration of the wave form inaccordance with which the volume oscillator is activated.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an assembly 11 for the in vivo measurement of various bloodparameters, and particularly the pH value and the concentrations(partial pressures) of oxygen and carbon dioxide. Although the assembly11 can be of different constructions, in this embodiment it includes asolution introducing system 13 and a probe-catheter assembly 15. Theassembly 11 may also include an instrument 17 for providing a readout ofthe blood parameters of interest.

Generally, the solution introducing system 13 introduces an appropriateanti-clotting solution, such as a heparinized saline solution, throughthe probe-catheter assembly 15 to the patient to keep the line leadingto the patient patent. Although this can be accomplished in differentways, in the embodiment shown schematically in FIG. 1, the system 13includes a pressurized source 19 of heparinized saline solution, aconduit 21 leading from the source to the probe-catheter assembly 15, aflow restrictor 23 to reduce the rate of flow through the conduit 21 tothe desired drop rate, a flush valve 25 in a bypass 27 around therestrictor 23, a stop cock 28, a volume oscillator, shown generally at29, a blood withdrawal site 30 and a pressure transducer 31. Except asnoted elsewhere herein, all of the components of the system 13 may beconventional, and the system 13 may include other components, ifdesired. In the illustrated embodiment, solution from the pressurizedsource 19 flows through the restrictor 23 at a relatively slow rate,such as 2 to 4 ml/hour. The solution flows through the volume oscillator29 and the probe-catheter assembly 15 to the patient. If a more rapidflow rate from the source 19 is desired, as for example during priming,the flush valve 25 can be manually opened to provide a relativelyhigh-rate flow path around the restrictor 23 in a conventional manner.

The pressure transducer 31 communicates with the conduit 21 and canmeasure the pressure therein. Accordingly, with the probe-catheterassembly 15 inserted into the vascular system of a patient, the pressuretransducer 31 can provide blood pressure readings. Volume oscillator 29is structured and operates, as discussed in detail hereinafter, to havelittle or no detrimental effect on the blood pressure readings providedby pressure transducer 31. The blood withdrawal site 30 is used fortaking blood samples from the patient through the probe-catheterassembly 15. The stop cock 28 is located proximally of the site 30 sothat, by closing the stop cock 28, the flush solution in the conduit 21cannot be withdrawn during a blood withdrawal procedure.

The probe-catheter assembly 15 includes a catheter 53 and a probe 55(FIG. 3). The catheter 53 may be a conventional arterial catheter. Assuch, the catheter 53 may include a proximal end 57, a distal end 59, alumen 61 extending axially, completely through the catheter and openingat a distal opening 63 at the distal end. The catheter 53 has a standardlead-in taper, i.e., a tapered zone 65, which extends from a referenceplane 66 along the outer periphery of the catheter 53 to the distal end59. The diameter of the lumen 61 also decreases distally throughout thetapered zone 65 as shown in FIG. 2. The catheter 53 has an externallythreaded coupling 67 at its proximal end.

The probe 55 may be of various different constructions. In theembodiment illustrated, probe 55 includes an oxygen sensor 69, a carbondioxide sensor 71 and a pH sensor 73, with each of the sensors orsensing elements affixed to the distal ends (optical surfaces) of singleoptical fibers 75, 77, and 79, respectively (FIG. 4). In thisembodiment, the sensors 69, 71 and 73 are fluorescent optical sensors,and they respond to the concentration of oxygen, the concentration ofcarbon dioxide and the pH value, respectively, to provide continuousoptical signals indicative of the condition sensed. The optical fibers75, 77 and 79 serve as transmission members for transmitting the signalsfrom the associated sensors proximally. The probe 55 has a very smallcross-sectional area. For example, the average transverse dimension, ordiameter, of the probe 55 within the lumen 61 perpendicular to thelongitudinal axis of the probe is preferably about 0.56 mm or less sothat it fits within the lumen with an ample radial clearance 81 as shownin FIG. 2. A probe having the specific small size noted above isparticularly useful when it is to be placed in a conventional 20 gaugeintra-arterial catheter, such as a 20G Deseret Insyte polyurethanecatheter. Both the surface of the catheter 53 and the portion of theprobe 55 within the lumen 61 may be coated or otherwise associated withheparin or the like anti-clotting component. The present system reducesthe risks of thrombus and clot formation so that excellent results, forexample, in terms of sensing accuracy and biocompatibility, are achievedwithout so treating such surfaces, in particular the catheter surface,with such anti-clotting component.

The sensors or sensing elements 69, 71 and 73 are each located away fromthe proximal end 57 of catheter 53, about 6 mm proximal of the distalopening 63 of the lumen 61. By locating the sensors 69, 71 and 73 inthis manner it has been found that, with volume oscillator 29 inactive,the sensors are exposed substantially exclusively to flush solution evenin the "worst" case in which the patient's heartbeat and otherenvironmental factors combine to cause blood from a human patient'sblood vessel to enter the opening and extend proximally into lumen 61.Also, with the sensors 69, 71 and 73 positioned in this manner, volumeoscillator 29 controls completely the time sequence in which blood isexposed to the sensors. It should be noted that with larger catheters,the sensing elements can be located somewhat more distally and stillachieve certain of the benefits of the present invention. For example,with a conventional 18 gauge catheter, the sensing elements can belocated within about 2.4 mm of the distal opening of the lumen. Toreduce the risk of blood stagnation and thrombus formation, it ispreferred that the sensing elements 69, 71 and 73 be located away fromthe proximal end 57 of catheter 53, more preferably at least about 10 mmdistal of the proximal end of the catheter.

The volume oscillator 29 can be preprogrammed with a wave form,preferably other than a simple sine wave, which controls the timesequence in which the sensors 69, 71 and 73 are exposed to substantiallyexclusively blood, and to substantially exclusively flush solution. Thiscontrol, which is substantially independent of the heartbeat of thepatient in whose cardiovascular system catheter 53 is located and of allother components of assembly 11, is an important aspect of the presentinvention and facilitates obtaining accurate blood parametermeasurements.

The particular design of the probe 55 forms no part of this inventionbecause this invention is applicable to probes of various differentconstructions. Briefly, however, the sensors 69, 71 and 73 are attachedto the distal ends of the associated optical fibers 75, 77 and 79 in anysuitable manner, and each of the sensors and the associated fiber isseparately encased in an inner overcoat 83 which, among other things,may assist in retaining the sensor on the end of the associated fiber.The overcoat 83 is, of course, permeable to the relevant bloodparameters so that such parameter, or one related to it, can be sensedby the sensors. An outer overcoat 85 covers the inner overcoats 83 and alength of the fibers just proximally of the overcoats 83. Outercovercoat 85 may, and preferably does, form a smooth distal tip of probe55 which reduces the risk of thrombus formation. Proximally of theovercoat 85, the optical fibers 75, 77 and 79 and atemperature-sensitive element, such as a thermocouple 86 (FIG. 4), aresuitably encased within an appropriate sheath 87.

The probe 55 includes a "Y" fitting 93 at its proximal end as shown inFIG. 3. The optical fibers 75, 77 and 79 extend within the sheath 87completely through one leg 95 of the "Y" fitting 93 to the instrument17, as shown in FIG. 1. Another leg 97 of the fitting 93 has a passage99 which communicates with the lumen 61, and more particularly, with theclearance 81 around the probe 55. The leg 97 is coupled to the conduit21 of the system 13 as shown in FIG. 1. A third leg 101 of the "y"fitting 93 carries a rotatable internally threaded coupling 103 forattaching the "y" fitting of the probe 55 to the proximal end 57 of thecatheter 53 outside the cardiovascular system of the patient.

Although the details of the fitting 93 form no part of this invention,the sheath 87 may be guided in the leg 95 by a sleeve 105 and retainedin position by potting 107. The sheath 87 extends within a flexible tube109 suitably attached to the leg 95, and shrink tubing 111 is providedover the adjacent end portions of the fitting and the tube for strainrelief.

With the proximal end 57 of the catheter 53 coupled to the probe 55 bythe coupling 103, the distal portion of the probe is within the lumen61, and the sensors 69, 71 and 73 are within the lumen properlypositioned relative to the distal opening 63, as shown in FIG. 2. Withthe catheter 53 within the cardiovascular system of the patient, such asin a radial artery, the catheter keeps the sensors 69, 71 and 73 fromcontacting the wall of the artery.

Referring now to FIGS. 6 to 9, volume oscillator 29 includes a holder121, a housing 123, a piston stem 125 and a piston end surface 127.Holder 121 includes a centrally located longitudinal cavity 129 which issized and adapted to receive housing 123, and laterally extendingcavities 131 and 133 sized and adapted to receive first extension 135and second extension 137, respectively, of the housing. Longitudinalcavity 129 includes an upwardly extending portion 139 adapted toaccommodate the movement of piston stem 125 into and out of housing 123.Holder 121 may be made of any suitable material, such as a suitablepolymeric material.

Housing 123 is adapted to be removeably secured to holder 121. Althoughvarious different constructions may be employed, the embodimentillustrated involves a peripheral groove or recess 141 in thecylindrically shaped sidewall 143 of housing 123. Holder 121 includestwo threaded holes 145 and 147 which pass through holder 121 andterminate at cavity 129. Conventional spring/ball assemblies 149 and 151are threaded into holes 145 and 147, respectively. The balls of theseassemblies are biased to extend into cavity 129. Housing 123 can beplaced in cavity 129 only by placing the balls of assemblies 149 and 151into recess 141. The biasing of these balls holds housing 123 in placein cavity 129, as shown in FIG. 7. Housing 123 can be convenientlyremoved from holder 121 by manually overcoming this biasing.

Housing 123 defines a flow path 153 which extends through extensions 135and 137 and provides fluid communication between conduit 21 and lumen61. When volume oscillator 29 is inactive, piston end surface 127 isstationary relative to the housing 123 and preferably substantiallyflush with the wall of flow path 153 so that no flush solutionaccumulates in volume oscillator 29.

The enlarged end 155 of piston stem 125 opposite piston end surface 127is adapted to be removeably secured or coupled to the shaft 156 of astepping motor 157, which is carried by holder 121. Alternately, aproportional solenoid can be used in place of stepping motor 157.Although various different constructions may be employed, the embodimentillustrated includes a shaft extension 159 secured to shaft 156.Extension 159 includes an open ended socket-like element 161 sized andadapted to receive and hold the enlarged end 155 of piston stem 125.This coupling is secure during use of volume oscillator 29, but can beeasily overcome by manual force when it is desired to remove housing 123and piston stem 125 from holder 121. 0-ring seal 163 is positioned tokeep the flow path 153 and chamber 170 hermetically sealed. 0-ring seal165 is positioned so that any fluid spills are prevented from contactingthe motor 157. Housing 123 and piston stem 125 are disposable, forexample, after use with a single patient, while holder 121 (and theother components of volume oscillator 29) are used repeatedly with manypatients. Since no blood comes into contact with the holder 121, thereis no cross contamination between patients.

Housing 123 and piston end surface 127 together form an open endedgenerally cylindrically shaped chamber 170 the volume of which variesdepending on the position of surface 127. When volume oscillator 29 isinactive, the volume of chamber 170 is substantially zero.

In the embodiment illustrated, as shown in FIG. 9, stepping motor 157 isdirectly secured to holder 121. Holder 121, in turn, is secured to apanel 172, which may be conveniently placed on a conventional IV poleplaced beside the patient. The pressure transducer 31 may also bemounted on the panel 172.

Stepping motor 157 has a "keyed" lead screw to provide linear motion tothe motor shaft 156, and normally operates so that the shaft moves inindividual steps of a given magnitude. In order to reduce the effectthat the volume oscillator 29 has on the other components of the system11, and in particular on the blood pressure readings provided bypressure transducer 31, it is preferred that the stepping motor 157 becontrolled so as to at least reduce the magnitude of these individualsteps. More preferably, the motor 157 is controlled so that the motorshaft 156 moves substantially smoothly, that is substantially withouttaking individual steps.

The motor 157 is controlled by a system in which control information isimputed from a control module 179 to a microprocessor 177 which, inturn, processes this information in accordance with one or morerelationships, e.g., a wave form through which the piston stem 125 is tobe moved, previously programmed into the microprocessor. This processingresults in control signals being passed from the microprocessor 177 toelectronics 173, including, for example, digital/analog convertors,power amplifiers and related components, to control the action of themotor 157.

As shown in FIG. 9, such electronics 173, termed micro-step electronicsto connote the "smoothing out" function of such electronics, are securedto panel 172 and are in electrical communication with stepping motor 157through electrical conduit 175. Also secured to panel 172 is themicroprocessor 177 and the control module 179.

FIG. 5 schematically illustrates how the movement of piston stem 125 iscontrolled. Control module 179 is in electrical communication, throughconduit 181, with microprocessor 177, which can be of conventionaldesign. Information, for example, the amount of time the piston stem 125is to take to move through one complete cycle (the cycle time) andwhether the volume oscillator 29 is to be continuously or intermittentlyactive, is manually imputed into microprocessor 177 through controlmodule 179, for example, through a series of touch keys, switches andthe like on the control module. Microprocessor 177 processes this inputin accordance with one or more relationships, for example, the wave formthrough which piston stem 125 is to move in one complete cycle,previously programmed into the microprocessor. Control instructions, inthe form of electrical signals, are then given by microprocessor 177 tomotor control (or micro-step electronics) 173 through conduit 183 which,in turn, controls the action of motor 157 and the movement of pistonstem 125.

The position of the piston stem 125 varies the volume of chamber 170 inaccordance with a wave form, preferably other than a simple sine wave,which wave form is preprogrammed into the microprocessor 177, usingconventional microprocessor programming techniques.

In use of the assembly 11, the catheter 53 is first inserted into theradial artery using conventional techniques. Next, the probe 55 isinserted into the lumen 61 and attached to the proximal end of thecatheter 53 with the coupling 103. This properly positions the sensors69, 71 and 73 within the lumen 61 about 6 mm from the distal opening 63.Conventional priming techniques may be employed to provide the desiredcontrolled flow of the flush or drip solution from source 19 into thelumen 61.

With volume oscillator 29 inactive, the flush or drip solution from thesource 19 completely fills the lumen 61 around the probe 55. Thesolution is provided under a pressure such that there is a slow flow ofsolution from the lumen 61 into the patient's artery. This introductionof the solution through the lumen 61 and into the artery results in aninterface zone 113 adjacent the distal opening 63 which has some axiallength and which includes both blood and the solution from the source19. The interface zone 113 is a partition between substantiallyexclusively blood distally of the interface zone and substantiallyexclusively solution, e.g., anti-clotting solution, from source 19,proximally of the interface zone. One important feature of the presentinvention involves at least reducing, or even minimizing, the amount (orfraction) of time that the sensors are exposed to this interface zone113 while the volume oscillator 29 is active, for example, relative to avolume oscillator which functions in accordance with a simple sine waveform. Also, in accordance with the present invention, the sensors 69, 71and 73 are exposed to substantially exclusively solution from source 19when the volume oscillator 29 is inactive.

with the volume oscillator 29 inactive, the interface zone 113 movesaxially back and forth near the opening 63 of the lumen 61 in a tidalaction as a result of the rising and falling of the patient's bloodpressure with each heartbeat. However, with the volume oscillator 29inactive, the interface zone 113 does not move sufficiently proximallyof the opening 63 to contact the sensors 69, 71 and 73. Thus with thevolume oscillator 29 inactive, the sensors 69, 71 and 73 are insubstantially exclusively the solution from source 19. This reduces therisk of thrombus formation at the tip of the sensors. In addition, withthe sensors 69, 71 and 73 located substantially exclusively in thesolution from source 19, the sensors can be effectively calibrated. Forexample, the carbon dioxide concentration of a typical heparinizedsaline solution from source 19 is substantially zero. Thus, the carbondioxide sensor 71 can be very effectively calibrated when the sensor islocated in substantially exclusively the solution from source 19.Similar calibrations can be obtained for the oxygen sensor 69 and forthe pH sensor 73 because the oxygen concentration and the pH of thesolution from source 19 are known. Moveover, these calibrations can beroutinely checked and rechecked when the sensors are exposedsubstantially exclusively to the solution from source 19, for example,when volume oscillator 29 is inactive.

In addition, with the volume oscillator 29 inactive, the pressuretransducer 31 can very effectively monitor the blood pressure of thepatient in whose cardiovascular system the catheter 53 is located.

Information is manually imputed through control module 179 tomicroprocessor 177 to control the activation of volume oscillator 29.Control module 179 may include one or more visual displays, for example,made up of liquid crystal displays, light emitting diodes and the like,which provide a visual display of one or more portions of theinformation, for example, the cycle time, imputed through the controlmodule. As discussed above, microprocessor 177 processes thisinformation in accordance with a preprogrammed wave form and providesinstructions to electronics 173 to control the action of motor 157 and,thereby, the movement of piston stem 125.

When volume oscillator 29 is activated, piston stem 125 is caused tomove in accordance with this imputed information and the preprogrammedwave form, which preferably is other than a simple sine wave. During atleast one portion of the cycle or wave form through which the pistonstem 125 moves, chamber 170 increases in size or volume so as to drawblood from the blood vessel of the patient back into the lumen 61 ofcatheter 53 so that the sensors 69, 71 and 73 are located in or exposedto substantially exclusively such blood. Preferably, when the volumeoscillator 29 is activated, the sensors 69, 71 and 73 provide parametersensitive signals to the instrument 17 so that values of the parametersof interest can be determined and monitored. Such signals are provided,preferably continuously provided, throughout the time the volumeoscillator 29 is active. Such signals may also be provided during atleast a portion of the time when the volume oscillator 29 is inactive.

It is preferred that the initial action of the piston stem 125 be suchso as to draw blood relatively quickly into the lumen 61 so that thesensors 69, 71 and 73 are quickly exposed to substantially exclusivelyblood. This reduces, or even minimizes, the time during which the bloodis exposed to the interface zone 113. The velocity at which blood isdrawn into and expelled from the lumen 61 should be tempered ormoderated so as to avoid damaging the blood itself. Since the system 11may be employed for relatively long periods of time, for example, on theorder of about 72 hours or longer, with blood being drawn into andexpelled from the lumen 61 many times, it is important that the systemhave no substantial detrimental effect on the blood itself.

For an intermediate period of time during the movement cycle of thepiston stem 125, the volume oscillator 29 preferably operates so as tomaintain the sensors 69, 71 and 73 in substantially exclusively blood.During this time, because the drip solution is continuing to be pumpedfrom source 19, the piston stem 125 may continue to be moved so as toincrease the volume of chamber 170. This increase in volume of chamber170 is preferably predetermined so as to accommodate the amount ofsolution from source 19 which would have flowed into lumen 61 duringthis period of time so that the blood level in the lumen is maintainedsubstantially constant.

After this intermediate period of time, which preferably is at least amajor portion and more preferably at least about 70%, of the total timerequired for a single cycle of the piston stem 125 , the piston stem ispreferably moved so as to push blood from the lumen 61 of the catheter53 out through the distal opening 63. This movement is done relativelyquickly. Again, care should be taken to avoid too rapid movement ofblood through the distal opening 63 so as not to damage the blood.However, once substantially all the blood has been removed from thecatheter 53, the rate of travel of the piston stem 125 is preferablyincreased so as to provide a fast or rapid flush of the tip of thesensors 69, 71 and 73 with the anti-clotting solution from source 19.This flushing action reduces the risk of thrombus formation at the tipof probe 55. At the end of this "flush" period, the cycle of the pistonstem 125 is completed and the volume of chamber 170 is substantiallyzero. This cycle can be repeated, if desired.

It should be noted that the relatively rapid movement of blood into andout of the lumen 61 reduces, and even minimizes, the amount of timeduring which the sensors 69, 71 and 73 are exposed to the interface zone113, which often has ambiguous and uncertain concentrations of theconstituents of interest. In so doing, the amount of time during whichthe .sensors 69, 71 and 73 are exposed to such ambiguous concentrationsof the constituents of interest is reduced or minimized. This featureincreases the accuracy of the present system.

Since the wave form used to move the piston stem 125 and the position ofthe sensors 69, 71 and 73 in lumen 61 are known, it is easily determinedwhat fraction of the cycle time of the piston stem 125 that the sensors69, 71 and 73 are exposed to substantially exclusively the solution fromsource 19. Thus, by factoring out this fraction of the cycle timemultiplied by the concentration of the constituent of interest in thesolution from source 19 (which is a known concentration) from theaverage value of the parameter of interest monitored by instrument 17during the time volume oscillator 29 is activated, one can veryconveniently and accurately determine the value of the parameter ofinterest in the blood in the radial artery of the patient.

Because the wave form of the movement of the piston stem 125 ispreferably chosen so that when the sensors 69, 71 and 73 are locatedsubstantially exclusively in blood the level or the amount of blood inthe catheter 155 is maintained substantially constant, the bloodpressure readings monitored by the pressure transducer 31 during thisperiod of time are substantially unaffected by the volume oscillator 29even though the volume oscillator is active or activated. Thus, littleor no interference with blood pressure monitoring results from theaction of the volume oscillator 29.

The following non-limiting example illustrates certain aspects of thepresent invention.

EXAMPLE

The system 11 is operated to monitor the carbon dioxide, oxygen and pHlevels of the blood in the radial artery of a patient. The catheteremployed is a 20G Insyte catheter. The probe 55 has an average diameterof 0.51 mm. The rate at which heparinized saline solution from source 19enters lumen 61 is 3.5 ml/hr.

The piston stem 125 is operated in accordance with a wave form showngraphically as 180 in FIG. 10. This wave form is expressed in terms ofthe volume (microliters or uL) of chamber 170 versus time. The flow ofblood into and out of the lumen 61 is shown at 182. This is expressed interms of volume (microliters or uL) versus time. The cross hatchedportion 184 of FIG. 10 represents the interface zone 113. Thus, thatarea of FIG. 10 above portion 184 can be considered substantiallyexclusively blood, while that area below portion 184 can be consideredsubstantially exclusively solution from source 19. Throughout the timethe piston stem 125 is activated, sensors 69, 71 and 73 continuouslyprovide parameter sensitive signals to instrument 17 indicative of thevalues of the respective parameters of interest.

As shown in FIG. 10, the piston stem 125 is initially moved relativelyquickly to increase the volume of chamber 170 so as to expose thesensors 69, 71 and 73 to substantially exclusively blood. This is doneby bringing into the lumen 61 only 5 uL of blood during the first secondat a "safe" (from the standpoint of avoiding damage to the blood)velocity of 5 uL/second. For the next seven seconds, the piston stem 125continues to move so as to maintain about 5 uL of substantiallyexclusively blood in lumen 61. During this time, the drip solution fromsource 19 is accommodated by a compensating expansion of the volume ofchamber 170. After this period of time, the blood is expelled from lumen61 at the "safe" velocity of 5 uL/second for the next second. Finally,the drip solution that was stored in chamber 170 is quickly expelled inthe last second to give the sensor tip with a "fast flush". This 10second cycle then repeats, as shown in FIG. 10. This cycle can berepeated continuously, intermittently or on demand, as desired and asimputed into the control module 179.

The values of the parameters of interest in the blood in the radialartery of the patient can be accurately determined from the averagevalues provided by instrument 17 while volume oscillator 29 is active.This is done, as described above, by factoring out from these providedaverage values the contribution to such values obtained while thesensors 69, 71 and 73 are exposed to substantially exclusively thesolution from source 19. To illustrate, assume that the average carbondioxide concentration provided by instrument 17 while volume oscillator29 is active is 40 mm Hg, and the carbon dioxide sensor 71 is exposed tosubstantially exclusively solution from source 19 (containing no carbondioxide) for 20% of the cycle time of the volume oscillator. The carbondioxide concentration in the blood in the radial artery of the patient,in mm Hg, is equal to ##EQU1##

It has been found that this approach provides increased accuracy inmeasuring carbon dioxide concentrations and oxygen concentrations ofabout 90 mm of Hg. (partial pressure) or greater relative to, forexample, a system in which the sensors are exposed to substantiallyexclusively blood and the interface zone 113 throughout the periodduring which parameter sensitive signals are provided to instrument 17.

In addition, the blood pressure monitored by pressure transducer 31 issubstantially unaffected by the action of the volume oscillator 29,particularly during the above-noted seven second period. Further,because of the relatively smooth functioning of stepping motor 157, evenwith blood being moved into and out of lumen 61, the blood pressuremeasurements of pressure transducer 131 are relatively unaffected by thevolume oscillator 29.

The above-noted cycle is repeated except that 10 uL of substantiallyexclusively blood is introduced into the lumen 61 instead of 5 uL ofsubstantially exclusively blood, as described above. Comparable resultsare obtained.

The present system has been found to provide accurate and reliabledeterminations of the values of parameters of interest in blood. Suchdetermination accuracy is quite unexpected since the sensors arepreferably exposed to substantially exclusively a non-blood-containingfluid medium for a portion of the time the sensors provide parameterssensitive signals. Such determinations are provided in a relativelystraight forward manner with little or no interference with othercomponents, for example, the pressure transducer, of the present system.The risk of thrombus formation is reduced, for example, by introducingonly a relatively small amount of blood into the lumen and/or byexposing the sensors to substantially exclusively a non-blood-containingfluid medium while the volume oscillator is inactive and/or by "fastflushing" the sensor tip when the volume oscillator is active. Inaddition, because the risk of thrombus formation is reduced, the presentsensing system may be employed in vivo for a longer period of time, forexample, on the order of about 72 hours or longer, relative to prior artsystems where thrombus formation and other factors rapidly reducedsystem accuracy. In effect, the present system has enhancedbiocompatibility. Further, the present volume oscillator system ispreferably partially disposable so that the advantages of the presentsystem can be achieved in a cost effective manner while maintainingpatient safety and isolation.

While this invention has been described with respect to various specificexamples and embodiments, it is to be understood that the invention isnot limited thereto and that it can be variously practiced within thescope of the following claims.

What is claimed is:
 1. An assembly for the sensing of a blood parametercomprising:a sensing element for sensing a parameter of blood andproviding a signal in response thereto, and an elongated transmissionmember for transmitting said signal from said sensing element; acatheter having a proximal end, a distal end, and a lumen extendingtherethrough, said lumen having an opening in said distal end, saidcatheter being sized and adapted so that at least said distal end andsaid opening are receivable within a blood vessel of a patient, saidsensing element being located within said lumen at a position away fromsaid proximal end of said catheter and at least 4 mm proximal of saidopening, said catheter being configured such that said catheter iscapable of being connected to a flush solution source so as to allow forthe introduction of flush solution into said lumen; a volume oscillatorin fluid communication with said lumen, said volume oscillator beingadapted to periodically cause blood from the blood vessel of the patientto move into and out of said lumen such that said sensing element isexposed to substantially exclusively blood for a portion of the timesaid volume oscillator is activated and to substantially exclusivelysaid flush solution for another portion of the time said volumeoscillator is activated; and means for calculating the value of saidblood parameter based upon both the values sensed by said sensingelement when exposed to substantially exclusively blood and when exposedto substantially exclusively said flush solution.
 2. The assembly ofclaim 1 wherein said sensing element is located at least 6 mm proximalof said opening.
 3. The assembly of claim 1 wherein said sensing elementis located up to 40 mm proximal of said opening.
 4. The assembly ofclaim 1 wherein said sensing element is located up to 20 mm proximal ofsaid opening.
 5. The assembly of claim 1 wherein said sensing element islocated closer to said opening than to said proximal end of saidcatheter.
 6. The assembly of claim 1 wherein said sensing element isadapted for sensing carbon dioxide.
 7. The assembly of claim 1 whereinsaid catheter has a longitudinal axis and an average diameter withinsaid lumen perpendicular to said longitudinal axis of about 0.56 mm orless.
 8. The assembly of claim 1 wherein said volume oscillator ispreprogrammed to periodically cause blood to enter said lumen and toexit said lumen in a predetermined time sequence.
 9. The assembly ofclaim 1 wherein said volume oscillator includes a housing and a piston,which are adapted to be disposed of after use with a single patient, andan actuator adapted to be removably coupled to said piston.
 10. Theassembly of claim 1 wherein said sensing element is adapted for sensingoxygen.
 11. An assembly for the sensing of a blood parametercomprising:a sensing element for sensing a parameter of blood andproviding a signal in response thereto, and an elongated transmissionmember for transmitting said signal from said sensing element; acatheter having a proximal end, a distal end, and a lumen extendingtherethrough, said lumen having an opening in said distal end, saidcatheter being sized and adapted so that at least said distal end andsaid opening are receivable within a blood vessel of a patient, saidsensing element being located within said lumen, said catheter beingconfigured such that said catheter is capable of being connected to aflush solution source so as to allow for the introduction of flushsolution into said lumen; a volume oscillator is fluid communicationwith said lumen, said volume oscillator being adapted to periodicallycause blood from the blood vessel of the patient to move into and out ofsaid lumen such that said sensing element is exposed to substantiallyexclusively blood for a portion of the time said volume oscillator isactivated and to substantially exclusively said flush solution foranother portion of the time said volume oscillator is activated; andmeans for calculating the value of said blood parameter based upon boththe values sensed by said sensing element when exposed to substantiallyexclusively blood and when exposed to substantially exclusively saidflush solution.
 12. The assembly of claim 11 wherein said sensingelement is located at least 10 mm distal of said proximal end of saidcatheter.
 13. The assembly of claim 11 wherein said sensing element isadapted for sensing carbon dioxide.
 14. The assembly of claim 12 whereinsaid sensing element is adapted for sensing oxygen.
 15. A method ofsensing a parameter of blood comprising:providing a catheter having aproximal end, a distal end, and a lumen extending therethrough, saidlumen having an opening in said distal end, at least said distal end andsaid opening being positioned within a blood vessel of a patient;providing a sensing element for sensing a parameter of blood andproviding a signal in response thereto and an elongated transmissionmember for transmitting said signal from said sensing element, saidsensing element being located within said lumen at a position at least 4mm proximal of said opening; introducing a flush solution from a flushsolution source into said lumen; providing a volume oscillator which,when activated, moves blood from the blood vessel of the patient intoand out of said lumen; and obtaining said signal from said sensingelement while said sensing element is exposed to substantiallyexclusively blood, wherein said obtaining step further comprisesexposing said sensing element to substantially exclusively said flushsolution for a portion of the time said volume oscillator is activatedand obtaining said signal while said sensing element is exposed tosubstantially exclusively said flush solution.
 16. The method of claim15 wherein said obtaining step comprises exposing said sensing elementto substantially exclusively blood for a major portion of the time saidvolume oscillator is activated.
 17. The method of claim 15 furthercomprising the step of exposing said sensing element to substantiallyexclusively said flush solution when said volume oscillator is inactive,and further comprising the step of locating said sensing element atleast 6 mm distal of said proximal end of said catheter.
 18. The methodof claim 15 wherein said obtaining step comprises obtaining said signalfrom said sensing element which senses carbon dioxide.
 19. The method ofclaim 15 further comprising the step of preprogramming said volumeoscillator to periodically cause blood to enter said lumen and to exitsaid lumen in a predetermined time sequence.
 20. The method of claim 15wherein said introducing step comprises including an anti-clottingcomponent in said flush solution.
 21. The method of claim 15 furthercomprising the step of activating said volume oscillator so as to causeblood to rapidly enter said lumen, to expose said sensing element tosubstantially exclusively blood for a relatively long period of time,and to cause blood to rapidly exit said lumen without substantiallydetrimentally effecting the blood.
 22. The method of claim 15 whereinsaid introducing step further comprises introducing the flush solutioninto said lumen at an increased flow rate for a period of time afterblood exits said lumen.
 23. The method of claim 15 wherein saidobtaining step comprises obtaining said signal from said sensing elementwhich senses oxygen.
 24. A method of sensing a parameter of bloodcomprising:providing a catheter having a proximal end, a distal end, anda lumen extending therethrough, said lumen having an opening in saiddistal end, at least said distal end and said opening being positionedwithin a blood vessel of a patient; providing a sensing element forsensing a parameter of blood and providing a signal in response thereto,and an elongated transmission member for transmitting said signal fromsaid sensing element, said sensing element being located within saidlumen; introducing a flush solution from a flush solution source intosaid lumen; providing a volume oscillator which, when activated, movesblood from the blood vessel of the patient into and out of said lumen sothat said sensing element is exposed to substantially exclusively bloodfor a portion of the time said volume oscillator is activated and tosubstantially exclusively said flush solution for another portion of thetime said volume oscillator is activated; and obtaining said signal fromsaid sensing element while said sensing element is exposed tosubstantially exclusively blood and while said sensing element isexposed to substantially exclusively said flush solution.
 25. The methodof claim 24 further comprising the step of exposing said sensing elementto substantially exclusively said flush solution when said volumeoscillator is inactive.
 26. The method of claim 24 further comprisingthe step of activating said volume oscillator so as to expose saidsensing element to substantially exclusively blood and to substantiallyexclusively said flush solution at predetermined times, and furthercomprising the step of calculating the value of said blood parameterusing a known value of the parameter in said flush solution.
 27. Themethod of claim 24 wherein said obtaining step comprises obtaining saidsignal from said sensing element which senses carbon dioxide.
 28. Themethod of claim 24 further comprising the step of preprogramming saidvolume oscillator to periodically cause blood to enter said lumen and toexit said lumen in a predetermined time sequence.
 29. The method ofclaim 24 wherein said introducing step comprises including ananti-clotting component in said flush solution.
 30. The method of claim24 further comprising the step of activating said volume oscillator sosensing element to substantially exclusively blood for a relatively longperiod of time, and to cause blood to rapidly exit said lumen withoutsubstantially detrimentally effecting the blood.
 31. The method of claim24 wherein said introducing step comprises introducing the flushsolution into said lumen at an increased flow rate for a period of timeafter blood exits said lumen.
 32. The method of claim 24 wherein saidobtaining step comprises obtaining said signal from said sensing elementwhich senses oxygen.
 33. An assembly for the sensing of a bloodparameter comprising:a sensing element for sensing a parameter of bloodand providing a signal in response thereto, and an elongatedtransmission member for transmitting said signal from said sensingelement; a catheter having a proximal end, a distal end, and a lumenextending therethrough, said lumen having an opening in said distal end,said catheter being sized and adapted so that at least said distal endand said opening are receivable within a blood vessel of a patient, saidsensing element being located within said lumen; and a volume oscillatorin fluid communication with said lumen, wherein said volume oscillatorincludes a housing and a piston, which are adapted to be disposed ofafter use with a single patient, and an actuator adapted to be removablycoupled to said piston.
 34. A method of sensing a parameter of bloodcomprising:providing a catheter having a proximal end, a distal end, anda lumen extending therethrough, said lumen having an opening in saiddistal end, at least said distal end and said opening being positionedwithin a blood vessel of a patient; providing a sensing element forsensing a parameter of blood and providing a signal in response theretoand an elongated transmission member for transmitting said signal fromsaid sensing element, said sensing element being located within saidlumen; introducing a flush solution from a flush solution source intosaid lumen; providing a volume oscillator which, when activated, movesblood from the blood vessel of the patient into and out of said lumen;obtaining said signal from said sensing element while said sensingelement is exposed to substantially exclusively blood; and exposing saidsensing element to substantially exclusively said flush solution whensaid volume oscillator is inactive.
 35. A method of sensing a parameterof blood comprising:providing a catheter having a proximal end, a distalend, and a lumen extending therethrough, said lumen having an opening insaid distal end, at least said distal end and said opening beingpositioned within a blood vessel of a patient; providing a sensingelement for sensing a parameter of blood and providing a signal inresponse thereto and an elongated transmission member for transmittingsaid signal from said sensing element, said sensing element beinglocated within said lumen; providing a volume oscillator which, whenactivated, moves blood from the blood vessel of the patient into and outof said lumen; introducing a flush solution from a flush solution sourceinto said lumen, wherein said introducing step further comprisesintroducing the flush solution into said lumen at an increased flow ratefor a period of time after blood exits said lumen; and obtaining saidsignal from said sensing element while said sensing element is exposedto substantially exclusively blood.